score = input('Enter score: ')

try:
    score = float(score)
    if score > 1.0 or score < 0.0:
        raise 'Bad score'

    if score >= 0.9:
        print('A')
    elif score >= 0.8:
        print('B')
    elif score >= 0.7:
        print('C')
    elif score >= 0.6:
        print('D')
    else:
        print('F')
except Exception as e:
    print('Bad score')
